Frequency modulator



Nov. 21, 1950 J. R. BOYKlN FREQUENCY MODULATOR Filed June 25. 1946 INVENTOR John IE. 50 y/Gh- WITNESSES:

Patented Nov. 21, 1950 UNITED STATES PATENT OFFICE 2,530,937 FREQUENCY MODULATOR John R. Boykin, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Application June 25, 1946, Serial No. 679,103

6 Claims. (01. 332-273 My invention relates to transmission systems for generating frequency modulated waves and. in particular, relates to a circuit employing a novel method of modulating frequencies by varying the equivalent reactance of a resonant circuit. My arrangement is, in fact, generally applicable to the problem of varying the tuning of resonant circuits for any purpose.

In transmission systems employing frequencymodulation of the carrier waves, one of the best known arrangements of the prior art has effected frequency-modulation of an oscillation generator by means of reactance tubes; that is to say, tubes which carried current substantially 90 degrees out of phase with the voltage across their terminals and in which the amplitude of such dephased current is varied in response to fluctuations of a signal Voltage impressed on control grids in the reactance tubes. However, this reactance tube method of frequency-modulating the oscillations generated hasdisplayed a practical defect of no small moment in that the control electrodes of the reactance tubes have been found to be subject to mechanical vibrations or microphonics. My invention overcomes this defect by employing in the resonant circuit, which determines the frequency generated by an oscillation generator, only diodes; i. e., tubes which contain no control electrodes. Such tubes are much less subject to difficulty from microphonic and mechanical vibrations than are triodes and other tubes containing control elec-,

trodes.

One object of my invention is, accordingly, to produce a system for frequency-modulating an oscillation generator which shall be substantially free from the effects of microphonics and mechanical vibrations.

Another object of my invention is to produce an arrangement for varying the resonant frequency of a tuned circuit by employing equipment which is not susceptible to the effects of shock, jar or microphonics.

Still another object of my invention is to provide a system for varying the reactance of an electric circuit by employing elements not subject to microphonic disturbances and the like.

Still another object of my invention is to provide an arrangement for varying the reactance of an electric circuit by employing tubes of the diode type.

Other objects of my invention will become apparent upon reading the following detailed description, taken in connection with the accompanying drawing, in which the single figure illustrates one form of circuit in which the prin- 2 ciples of my invention may conveniently be embodied.

The basis for my invention is the fact that where a capacitor is connected to an alternating voltage source in series with a variable resistor, the reactive component of the current flowing is found to vary with the changing value of the resistor; and such reactive component is found to vary most rapidly with changes of resistance when the resistance and the reactance are equal to each other.

The foregoing principle is applied to frequencymodulating the output of a transmitter in accordance with the drawings herein in which tubes i and 2 are grid-controlled oscillator tubes of any conventional type, being shown for purposes of illustration as pentodes. The respective anodes of the tubes I and 2 are connected to the outside terminals of a pair of seriallyconnected inductors 3 and 4 having their common terminal connected through a capacitor 5 to ground. Any suitable output circuit 6, in which it is desired to employ frequency-modulated oscillations, may be connected to the anodes of the tubes I and 2. Thetubes I and 2 have their suppressor grids connected to their cathodes and have their screen grids respectively connected to ground through capacitors I and 8. The cathodes of tubes I and 2 are interconnected with each other through a pair of inductors 9, It], the common terminal of which is connected to ground. The control electrodes of the tubes I and 2 are connected to each other through four serially-connected capacitors I2, I3, I4, I5. The common terminal of the middle pair of capacitors I3 and It is connected to ground, and the two other common terminals of the abovementioned four capacitors are connected to the cathodes of tubes I and 2. The control electrodes of the tubes I and 2 are likewise connected to ground through a pair of variable capacitors I'I, I8, and are likewise connected to each other through an inductor is havin its mid-terminal connected to ground through a resistor 20. The circuit so far outlined will be recognized by those skilled in the art as a wellknown type of oscillation generator system, the resonant frequency of the output current of which is fixed by the tuned tank circuit l'i, l8, IS.

The control electrodes of the tubes I and 2 are likewise connected to each other through a channel comprising a capacitor 2!, a diode 2.2, a second diode 23 and a capacitor 24, said diodes being oppositely poled. It will be evident that the diodes 22 are, in effect, resistors connected in series with the capacitors 2| and 24, and that variations in their resistance will, in accordance with the principles pointed out above, change the magnitude of the reactive component of the current drawn from the tank circuit I8, I 9 by the capacitors 2| and 24. The diodes 22 and 23 constitute the variable resistance which modify the amount of reactive current drawn from the above-mentioned tuned circuit by the capacitors 2| and 24. Varying the resistance of the diodes 22 and 23 thus varies the resonant frequency of the circuit l8, l9, 2|, 22, 23, 24, which fixes the frequency of the oscillations generated by the tubes and 2.

To controllably vary the resistance of the diodes 22 and 23, I connect the positive terminal 25 of a direct-current source having its negative terminal grounded to the respective anodes of the tubes 22 and 23 through resistors 26, 21. I connect the cathodes of the tubes 22 and 23 to the anode of an electron tube 28 having a cathode connected to ground through a capacitor 29. I connect the control electrode of the tube- 28 to ground through a resistor 3| shunted by a microphone 32 in series with a suitable voltage source 33. Between the terminal 25 and the cathodes of the diodes 22 and 23, I connect serially a pair of capacitors '34 and 35, and ground their common terminal. It will be noted that the channel comprising capacitors 2|, 35 and diode 22 is connected directly in shunt with capacitor l! to ground; and a similar statement applies to the homologous capacitors and diode 23 on the other side of the frequency determining circuit. The value of the resistance 26 is such that the reactance of the parallel networks 26, 22, 35, 34 is equal to the resistance 2| at the unmodulated frequency of the oscillators l and 2. Essentially this value is the equivalent reactance of resistor 20 and diode 22 in parallel since the capacitors 34 and 35 are by-pass capacitors to ground. Correspondingly the parallel networks 21, 23, 34, 35 has a resistance equal to the reactance of capacitor 24 at the unmodulated frequency of the oscillators.

It will be evident that signal voltages impressed by the microphone 32 on the control electrode of tube 28 will vary the amount of plate current drawn by that tube through the tube diodes 22 and 23. Variation of this direct current flowing through the diodes 22 and 23 obviously varies their internal resistance and so varies the resistances in series with the capacitors 2| and- 24. The resonant frequency of the circuit comprising elements I8, l9, 2|, 22, 23, 24 thus varies in accordance with instantaneous fluctuations of the voltage impressed on the grid of tube 28 by microphone 32. The frequency of the voltage generated in the tubes and 2 is thus modulated in accordance with the fluctuations of the signal voltage impressed by the microphone 32.

It will be obvious that the above-described arrangement involves extremely simple circuits and is economical to construct and operate. It will likewise be evident that while I have described the device as applied to an oscillation generator of a particular type, it may be applied to any type of oscillation generator making use of a resonant circuit. In particular, it may be applied to oscillation generators employing a single tube in place of the push-pull tube-pair 2, a single capacitor 2| in series with a single diode 22 then sufficing when shunted across a be evident that while I have described the resonant circuit as employed in an oscillation generator, it is of general application to many fields where resonant circuits are used in the alternating-current art. Furthermore, while the reactance which I vary forms a component in a resonant circuit, it will be evident that the device is likewise applicable to varying reactances in alternating-current circuits, generally. Likewise, the particular type of variable resistor, i. e., a diode which is employed in series with the reactance element may be replaced satisfactorily for many purposes by other known types of variable resistor; e. g., by a carbon-type microphone. Furthermore, while I have, in diodes 22 and 23, shown a particular type of rectifier, any other type of rectifier having a resistance varying with. the direct-current voltage impressed upon it, as a selenium or a copper oxide rectifier, may be substituted for such diodes.

I claim as my invention:

1. In combination, a generator of electrical oscillations, including a frequency determining network, a component having a non-linear, current-voltage characteristic, and an electric discharge device having a control electrode and a plurality of principal electrodes, said component and said principal electrodes being connected in. series with each other to said frequency determining network, and terminals for impressing a control potential between said control electrode and one of said principal electrodes.

2. The combination according to claim 1 in which the electric discharge device is a pentode.

3. The combination according to claim 1 in which the component having a non-linear, current-voltage characteristic is a diode.

4.. The combination according to claim 1 in which the component having a non-linear, current-voltage characteristic is a diode having its anode connected to the frequency determining network and its cathode to one of the principal I electrodes of the discharge device.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date,

2,296,962 Tunick Sept. 29, 1942 2,354,799 Crosby Aug. 1, 1944' 2,374,000 Crosby Apr. 17, 1945 2,430,126 Korman Nov. 4, 1947 2,436,834 Stodola Mar. 2, 1948 

